As observer devices for monitoring people, what are used are cameras, and in some resent cases, UWB (Ultra Wide Band) tags or the like. Normally, when those observer devices are to be used, calibration of the position of the observer devices must previously be performed. In performing the calibration of the position of the observer devices, generally, an artificially created marker is used. Example of the marker for the tag reader includes position tag having stored therein position information. Further, example of the marker for the camera includes two-dimensional barcodes having stored therein position information. Whichever the marker is used for, i.e., for the tag reader or for the camera, it necessitates labor such as storing the position information, and disposing the marker at that position.
On the other hand, there is a technique of using a known object existing in an environment to estimate the self-position of a robot instead of using such an artificial marker (Patent Document 1).